It has been documented that a digital-to-analog converter may be used to process digitized signals for amplification by an analog power amplifier. For example, Erik Bresch and Wayne T. Padgett in “TMS320C67-Based Design Of A Digital Audio Power Amplifier Introducing Novel Feedback Strategy” (http://www.ti.com/sc/docs/general/dsp/fest99/poster/hbreschpadgett.pdf) describe such an amplification technique used in connection with audio signals. FIG. 1 shows a basic structure of a Class-D type audio power amplifier as depicted by Bresch and Padgett.
Certain digital audio signal sources, such as compact disk (CD) players, provide digitized audio signals that are pulse code modulated. Such digitized audio signals may have a 16-bit resolution and a 44.1 kHz sampling frequency. However, the audio signal may need to be modulated and amplified to be useful in particular applications.
A number of techniques allow a digital data stream to be represented as an analog signal. One such technique includes the use of a sigma-delta modulator, and another technique includes employing a pulse width modulator.
Each of these two techniques has certain advantages and disadvantages for particular applications. For example, output from a sigma-delta modulator may have a high degree of accuracy such that the amount of noise is relatively low and the amount of total harmonic distortion (THD) is favorable (perhaps about 0.001%) for certain audio equipment applications. As documented by James Candy and Gabor Temes in “OverSampling Delta-Sigma Data Converters” (ISBN 0-87942-285-8), one conventional technique used in certain sigma-delta modulators is to convert a stream of 16-bit audio data into a stream of 4-bit data at a higher clock rate. It has also been documented that the noise associated with such a quantization to 4-bits may be “shaped” so that it all appears in relatively high frequencies. However, one disadvantage of such a technique (at least for certain audio equipment applications) is that digital output from a sigma-delta modulator may not be easily converted into an analog voltage due to variable frequencies present in the data stream.
A pulse width modulator may produce an output with a low and controlled frequency that may be able to drive a Class-D type audio power amplifier and be relatively easily converted into an analog voltage. In addition, certain pulse width modulation techniques may introduce less error than certain sigma-delta modulation techniques.
Certain developers have tried to produce a signal with the positive distortion and noise performance characteristics of a sigma-delta modulation technique, as well as the low frequency and predicable output characteristics of a pulse-width-modulation technique. For example, Bresch and Padgett have documented one such attempt in “TMS320C67-Based Design of a Digital Audio Power Amplifier Introducing Novel Feedback Strategy.” In addition, it has been documented by K. P. Sozaski, R. Strzelecki and Z. Fedyczak in “Digital Control Circuit for Class-D Audio Power Amplifier” that an attempt to combine a sigma-delta type modulator with a pulse width modulator resulted in a signal-to-noise ratio that approached 75 db in the audio frequency band (i.e., 20 Hz to 20 kHz). However, such performance may be insufficient or unacceptable in certain circumstances or for particular users.
Recognizing that a pulse width modulator may introduce distortion (which is thought to be caused by a high harmonics content in the audio frequency band) into a signal such as an audio signal, certain developers have tried to reduce the distortion by (1) using a sigma-delta-to-pulse-width-modulator circuit to create an analog output, and then (2) feeding back that analog output in a closed loop system to create an error signal. U.S. Pat. No. 6,515,604 to Delano discusses such a system to create an error signal. Bresch and Padgett discuss another system of this type in “TMS320C67-Based Design of a Digital Audio Power Amplifier Introducing Novel Feedback Strategy.”
For certain applications or users, however, it may be desirable to correct for distortion in the digital domain. In addition, it may be desirable at least for particular audio equipment applications to have a signal processor capable of a high degree of noise shaping similar to a sigma-delta modulator. Additional favorable characteristics of such a signal processor may include both a modulation depth such that large signal amplitudes may be generated in a particular frequency range (e.g., the audio frequency range), as well as the use of a relatively small and fixed number of values for a given time period such that a simple digital to analog converter can be made using, for example, an RC network connected to a simple digital driver. Yet another desirable characteristic of such a signal processor may be a low output frequency so that it can be used to drive switches (e.g., MOSFETs) of the type used in certain Class-D type audio power amplifiers.